To the OP,

Stop focusing on the chain length. As cny-bikeman said, you have the benefit of advice from people with something like 75-100 years of experience between them, telling you that the most likely issue is a bent or twisted hanger or a high jockey pulley (B-screw adjustment).

There are 3 ways to measure a chain, all equally valid, except that everybody has a favorite. Campagnolo suggests using the longest chain possible (no sag on small/small) Sram recommends the shortest (big/big +1") and Shimano is like the baby bear taking a middle ground position within their vertical cage method.

Note that if there's no extra RD take up capacity, all three methods yield the same answer, the minimum is the maximum and the middle option all at the same time.

Go back to the bicycle, shift to a problematic combination and look at the area where the chain leaves the upper pulley and meets the sprocket. There should be (must be) roughly 1" of free chain free between the two. If the pulley and sprocket are touching through the chain or early so you'll experience the type of problem you're having.

You correct that with the B-screw, but don't just turn it in, instead pull the RD back against the spring a bit to take the weight off, adjust the screw then let the RD swing forward. If adjusting the B-screw doesn't change anything, then possibly the stop cam is damaged or mismounted.

A bent hanger may also cause problems because the plane the RD cage rotates in isn't parallel to the plane of the bike. That means that as the jockey wheel will move inward (or out) as the cage rotates causing a change in trim.

From your description, I lean more to the high pulley (b-screw) theory, but take advantage of the advice you've been given, and check both the hanger and the pulley height. If both are correct, then something elso might be happening, and you should let a mechanic look and see if with the benefit of hands on he can solve it for you.